The present invention relates to sensing position and motion using sensor devices, and more specifically to sensing position and motion using optical encoder sensors.
Encoders are useful in many applications. In one application, computer interface devices such as joysticks, mice, track balls, steering wheels, etc. make use of encoders to determine the position of a user manipulatable object (manipulandum) in a workspace of the user object, and provide the position information to a host computer that is connected to the interface device. An encoder can be used to sense position of the manipulandum in one or more degrees of freedom. Haptic feedback interface devices are a form of interface device in which motion of the manipulandum is sensed and forces are output on the manipulandum or the device housing using actuators such as motors. Haptic feedback devices require accurate position sensing of a manipulandum to determine force output, especially force feedback devices which output forces on the manipulandum in its degrees of freedom.
Optical encoders use paired light sensors and light sources with mechanical interruptions to measure rotary or linear position. Rotary optical encoders are a widely-used form of digital sensor. Optical encoders used for sensing rotational motion typically include a spinning disk attached to a moving member such as a rotating motor shaft. For example, FIG. 1 is a diagram of an incremental rotary optical encoder 10 attached to a motor shaft 12 of a motor 14 to measure the angle of rotation of the shaft. A light source 16 emits a beam of electromagnetic energy toward a light sensor or detector 18 through an encoder disk 20 which is transparent or includes open slots. An encoder hub 22 couples the disk 20 to the shaft 12. Electronics 24 allows the necessary signals to pass to and from the source and detector. FIG. 2 shows the face of the optical encoder disk 20, with a striped pattern 26 of spokes or slits that provide periodic interruptions between the light source and light sensor as the motor shaft rotates. This creates a stream of pulses at the output of the detector.
The encoder 10 of FIG. 1 preferably uses two emitter-detector pairs (both emitters included in the light source 16 and both detectors included in the light sensor 18). If only one pair is used, the single output signal indicates motion, but cannot indicate which direction the encoder disk/shaft is turning. A practical encoder requires the addition of a second detector (and second emitter if appropriate) slightly offset from the first so that it produces a square wave pulse stream that is 90 (electrical) degrees out of phase with the first pulse stream, allowing the sensing of direction of motion, as is well known to those of skill in the art.
Optical encoders typically use LEDs to emit the light that passes through the encoder disk to the detector. While LEDs are an inexpensive, proven technology, they are far from ideal in many respects. The perfect optical encoder emitter would emit a bright, collimated light beam, and would require drive currents as low as practically possible. A bright emitter raises the signal-to-noise ratio of the sensor system. Collimated light projects a uniform pattern of shadows from the encoder disk that does not change size if the disk flutters closer to or farther from the emitter. Even distribution of light intensity simplifies the design of the detector. Low drive currents are especially important for battery-powered systems and devices running off of low-power buses such as those found in portable military and commercial systems.
Optical encoders use detectors to capture the light signals created by the emitter and rotating disk and convert them into electrical signals. After amplification and analog-to-digital conversion the TTL-compatible signal can be read by a microcontroller or data acquisition hardware. Two channel signals (one channel from each detector) are typical output of a ubiquitous quadrature optical encoder, using two photodetectors with channel signals offset by 90 degrees (¼ of the grid spacing interval on the encoder disk). High-resolution quadrature encoders, however, are typically large and expensive.
Optical encoder disks play a key role in determining the resolution and size of optical encoders. Higher resolution encoders require finer pitch grid patterns on the disks, or disks with larger circumferences which possess more grid lines per revolution. Manufacturers use various technologies to fabricate encoder disks. The cost and minimum grid pitch vary with the technologies. Most encoders use plastic film disks imprinted with standard lithographic methods. These disks can reliably accommodate a grid pitch as fine as 0.002″. The cheapest technology, molded plastic disks, can only achieve a grid pitch of 0.040″.
The performance of the optical encoder is determined by the characteristics of the emitter, the detectors, and the encoder disk. The required sensing resolution of the encoder largely determines the size of the encoder by influencing the size of the encoder disk. Making encoders smaller requires more precise fabrication of the encoder disk, better emitters, and more sophisticated detector arrays. However, any of these measures can become prohibitively expensive, especially for low-cost products in which the encoders are incorporated. For example, interface devices such as mice, joysticks, or the like could benefit greatly from smaller and more accurate optical encoders, but the cost of these devices must be kept low to remain competitive in the consumer market.